diff --git a/frontend/src/utils/__tests__/formatDateTimeLocalInput.spec.ts b/frontend/src/utils/__tests__/formatDateTimeLocalInput.spec.ts
new file mode 100644
index 0000000000..29b5be2703
--- /dev/null
+++ b/frontend/src/utils/__tests__/formatDateTimeLocalInput.spec.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from 'vitest'
+
+import {
+  formatDateTimeLocalInput,
+  getBrowserTimeZone,
+  parseDateTimeLocalInput
+} from '../format'
+
+describe('formatDateTimeLocalInput', () => {
+  it('round-trips a minute-precision local timestamp', () => {
+    const timestamp = Math.floor(new Date(2026, 7, 30, 7, 24).getTime() / 1000)
+
+    expect(formatDateTimeLocalInput(timestamp)).toBe('2026-08-30T07:24')
+    expect(parseDateTimeLocalInput('2026-08-30T07:24')).toBe(timestamp)
+  })
+})
+
+describe('parseDateTimeLocalInput', () => {
+  it('uses local date components and rejects timezone-bearing values', () => {
+    const expected = Math.floor(new Date(2026, 7, 30, 7, 24, 10).getTime() / 1000)
+
+    expect(parseDateTimeLocalInput('2026-08-30T07:24:10')).toBe(expected)
+    expect(parseDateTimeLocalInput('2026-08-30T07:24:10+08:00')).toBeNull()
+    expect(parseDateTimeLocalInput('2026-08-30T07:24:10Z')).toBeNull()
+  })
+
+  it('rejects empty, malformed, and overflowing calendar values', () => {
+    expect(parseDateTimeLocalInput('')).toBeNull()
+    expect(parseDateTimeLocalInput('2026-08-30 07:24')).toBeNull()
+    expect(parseDateTimeLocalInput('2026-02-30T07:24')).toBeNull()
+    expect(parseDateTimeLocalInput('2026-08-30T24:00')).toBeNull()
+    expect(parseDateTimeLocalInput('2026-08-30T07:60')).toBeNull()
+  })
+
+  it('accepts optional seconds and fractional seconds', () => {
+    const expected = Math.floor(new Date(2026, 7, 30, 7, 24, 10, 120).getTime() / 1000)
+
+    expect(parseDateTimeLocalInput('2026-08-30T07:24:10.12')).toBe(expected)
+  })
+})
+
+describe('getBrowserTimeZone', () => {
+  it('returns a timezone identifier or the UTC fallback', () => {
+    expect(getBrowserTimeZone()).toBeTruthy()
+  })
+})